Resonance Raman Study of Aggregated Meso-tetra(4-pyridinium) porphyrin Diacid

Resonance Raman spectra of aggregated meso-tetra(4-pyridinium)porphyrin diacid (H₈TPyP⁶⁺)were studied with excitation near the exciton absorption bands of 470 nm.The UV-Vis absorption and resonance light scattering spectra of H₈TPyP⁶⁺ monomers and aggregates were also measured. The observed Raman bands of monomeric and aggregated H₈TPyP⁶⁺ were assigned on the basis of the observed deuteration shifts and by comparing with the Raman spectra of analogous porphyrin diacids. Aggregation causes moderate downshifts(2～6 cm^-1) for high-frequency modes involving the in-plane CC/CN stretches of the porphyrin core and a dramatic upshift (12 cm^-1) for the out-of-plane saddling mode of the porphyrin ring. The structural changes induced by aggregation and the possible hydrogen bonding interaction between the H₈TPyP⁶⁺ molecules in the aggregate are discussed based on the spectral observations.